1. Field of The Invention
The present invention relates to a deflection plate for use in an optical pick-up device for reading an information from an optical disk.
2. Description of the Prior art
Optical disk systems, including compact disc (Trade Mark) systems, have become an increasingly important means of recording and storing information. In these systems, the optical pick-up devices are used to read the information from the recording medium.
An optical pick-up device typically consists of several optical components, including a semiconductor laser as the light source, a photo-diode as the detector, and lenses and prisms. Some of the recent optical pick-up devices are employed with a new deflection plate type signal detection method.
A conventional deflection plate type optical pick-up device is disclosed, for example, in U.S. Pat. No. 4,358,200 which is issued Nov. 9, 1988, Japanese Patent Laid-open Publication No. 64-4926 published Jan. 10, 1989, Japanese Patent Laid-open Publication No. 63-22340 published Sep. 16, 1988 or Japanese Patent Laid-open Publication No. 63-222341.
In FIG. 21, a conventional deflection plate-type optical pick-up device such as disclosed in Japanese Patent Laid-open Publication No. 56-57013, is shown wherein the laser beam Lp' emitted by the semiconductor laser chip 31, passing through the deflection plate 32, is converged by the lens 33 to focus a spot on the optical disk 34, from which the laser beam Lp is then reflected. The reflected laser beam Lp is converged again by the lens 34 and then deflected by the deflection plate 32 so as to focus on the servo detection detector 35.
In FIG. 22, another conventional deflection plate-type optical pick-up device, such as disclosed in Japanese Laid-open Patent Publication No. 64-4926, is shown. The deflection plate 42 has a first diffraction area 42a on one surface facing the laser diode 41 and a second diffraction 42b on the other surface thereof. The laser beam Lp emitted by the laser diode 41 is split into three beams Lp1, Lp2, and Lp3 having diffraction orders of "+1", "0", and "-1" by the first diffraction area 42a. The three beams Lp1, Lp2, and Lp3 are converged by the lens 43 so as to focus spots Sbp, Sap and Scp on the surface of optical disk 44, respectively. Three beams Lp1, Lp2, and Lp3 are impinged on the optical disk 44 and are reflected, through the lens 43, to the deflection plate 42 at which each of three beams is further split by the second diffraction area 42b into three beams having different diffraction orders. From such re-split beams, only three beams of "+1" diffraction order Lp1', Lp2', and Lp3' are led to the photo detector 45 so as to focus spots Sbp", Sap", and Scp" thereon. The laser spots Sbp" and Scp" are used for tracking error signals and spot Sap" is for a focus error signal.
In FIG. 23, a conventional deflection plate for an optical pick-up device, such as disclosed in Japanese Patent Laid-open Publication 63-222341 is shown. The deflection plate 52 is formed in a pattern in which the grated depth is varied from the center to the edges to provide arcade ribs in order to impart a particular optical filtering function.
In FIG. 24, another conventional deflection plate for an optical pick-up device, such as disclosed in Japanese Patent Laid-open Publication No. 63-222340 is shown. The deflection plate 62 is formed in a pattern in which the duty ratio is varied from the center to the edges in order to impart a particular optical filtering function.
However, there are several problems with the above described conventional deflection plates. According to the deflection plate shown in FIG. 22, two diffraction grating patterns are formed respectively on opposite surfaces of the deflection plate. The first pattern formed on the bottom surface when viewed in FIG. 22 is used for splitting the laser beam emitted by the laser unit into three beams, and the second pattern formed on the top surface in FIG. 22 is used for deflecting the reflected beams from the optical disk, toward the servo detection detector.
In addition, the optimum diffraction efficiency of "1" order of the first pattern for the three-beam generation and that of the second pattern for the deflection of the reflected beams differ such that optimum diffraction efficiency of "1" order of the latter pattern is, in general, greater than the former.
However, in the deflection plates illustrated in FIGS. 23 and 24, the diffraction efficiencies of the laser beam emitted from the semiconductor laser and the reflected laser beam from the recording medium are the same. Furthermore, although the deflection plate illustrated in FIG. 22 can have the different diffraction efficiencies of the laser beam emitted from the semiconductor laser and the laser beam reflected from the recording medium, it is necessary to form the diffraction deflection plate patterns on both sides, resulting in a complicated manufacturing process.